Relativistic Effects Stabilize Unusual Gold(II) Sulfate Structure via Aurophilic Interactions

The crystal structure of gold(II) sulfate is strikingly different from other coinage metal(II) sulfates. Central to the unsual AuSO₄ bulk structure is the Au₂⁴⁺ ion with a very close Au-Au contact, which is a structural feature that does not appear in CuSO₄ and AgSO₄. To shed some light on this unusual behavior, we decided to investigate the relative stabilities of the coinage metal(II) sulfates utilizing periodic Density Functional Theory. By computing relative energies of the hypothetical nonrelativistic gold(II) sulfate (Au_NRSO₄) in different structural arrangements and performing chemical bonding analyses employing the Electron Localization Function as well as the Quantum Theory of Atoms in Molecules method, we show that the stability of the unsual AuSO₄ bulk structure can be related to aurophilic interactions enabled by relativistic effects. From the relative stabilities and UV-vis spectra computed via GW methodology, we predict that Au_NRSO₄ would assume the structure of either copper(II) sulfate or silver(II) sulfate with almost equal likelihood and appear as bright-violet or deep-blue substances, respectively.